Visual transduction in vertebrate rods and cones comprises the sequence of events leading from photon absorptiion to change in the amount of electrical current flowing through the photoreceptor outer segment membrane. Rods and cones both contain a cGMP-phophodiesterase which is controlled by light, and cGMP is apparently the agent in both cells that controls the flow of current through the outer segment membrane. Experiments are proposed here to determine if the photocurrents of rods and cones can be accounted for quantitatively in terms of reactions thought to be components the cGMP enzyme cascade and in terms of the speed and voltage dependence of Na/Ca exchange. The hypothesis that one of the components of the cascade has a decay rate constant set by o.s. free calcium will be tested. The hypothesis that the 10-fold greater speed of cones arises because of the relatively greater speed of their Na/Ca exchange will be tested. The hypothesis that the state of light-adaptation of amphibian rods and cones is determined solely by outer segment free calcium will be tested. Three preparations will be used for these experiments: (1) toad rod disk membranes suspended in cuvette and assayed for light-activated phosphodiesterase with pH technique; (2) isolated single toad rod outer segments held in suction pipette and perfused; (3) isolated intact salamander rods and cones, held in suction pipette and infused with test substances through voltage-clamp pipette.